1. Field of the Invention
The invention relates generally to measurement-while-drilling apparatus used in earth boring operations. In various measurement-while-drilling apparatus information about downhole conditions is telemetered to the surface of the bore by means of pressure changes (xe2x80x9cmud pulsesxe2x80x9d) in the drilling fluid (xe2x80x9cmudxe2x80x9d) which is circulated through the drill sting and annulus of the bore during drilling. Such mud pulses are typically created by alternatively restricting and unrestricting the flow of mud through a downhole valve (xe2x80x9cmud valvexe2x80x9d). In various MWD systems actuation of the mud valve is accomplished through use of hydraulic system. The invention disclosed and claimed herein relates to an improved hydraulic system for actuation of an MWD mud valve.
2. Description of Related Art
In the field of earth boring, MWD apparatus are employed to obtain information about downhole conditions, such as wellbore inclination angle, magnetic heading, orientation of the bottom-hole assembly, formation radioactivity, resistivity and porosity without the necessity of interrupting drilling operations. To accomplish that end, MWD apparatus typically employ sensors located as close as practical to the drill bit, means to encode the sensed data into a pulse format, and means to telemetry the data pulses to the surface. A common means of telemetering such data to the surface is by causing back-pressure changes (xe2x80x9cmud pulsesxe2x80x9d) to occur in the drilling fluid (xe2x80x9cmudxe2x80x9d) which is circulated during drilling. In such means at least a portion of said drilling fluid is flowed through a mud valve located downhole, which valve is alternatively actuated between two positions (alternatively xe2x80x9crestrictingxe2x80x9d and xe2x80x9cunrestrictingxe2x80x9d the flow of drilling fluid through the mud valve). This causes corresponding changes in back-pressure (xe2x80x9cmud pulsesxe2x80x9d) of the drilling fluid. These mud pulses may be detected at the surface of the bore and decoded to retrieve information about downhole conditions.
In certain forms of MWD apparatus, actuation of the mud valve is accomplished by means of a hydraulic system. U.S. Pat. Nos. 4,266,606, 3,756,076, 3,737,843 and 3,693,428 disclose hydraulic systems for operation of an MWD mud valve. In these patents the piston of the linear hydraulic actuator is operatively coupled to the poppet of a poppet-seat type mud valve. When hydraulic pressure is applied to one side of the piston, the piston moves to one end of the actuator which causes the poppet to move towards the seat of the mud valve. Reducing the gap between the poppet and seat causes back pressure of drilling fluid to increase. Alternatively, when hydraulic pressure is applied to the opposite side of the piston, the piston moves to the other end of the actuator, increasing the gap between the poppet and seat, causing back pressure of the drilling fluid to decrease. No means is provided in these patents to by-pass hydraulic fluid through the actuator after its piston has stroked a desired amount. Rather in operation, full hydraulic pressure is continuously applied to either one or the other side of the piston.
U.S. Pat. No. 6,050,349 discloses a hydraulic system for operating a hydraulic actuator of rotary configuration. When hydraulic pressure is applied to one side of a vane type piston, the piston rotably moves to the opposite side of the actuator and closes a mud valve port, causing back pressure of the drilling fluid to increase. When hydraulic pressure is applied to the opposite side of the actuator, reverse movement and effect occurs. Similar to the above patents, the actuator of this patent contains no means to by-pass hydraulic fluid through the actuator after piston has stroked in either direction. Rather during operation hydraulic pressure continuously loads either one or the other side of the piston, continuously loading the hydraulic system.
Continuous loading of the hydraulic system constitutes a source of heat which can be degrade performance of the hydraulic system or damage equipment, particularly in high temperature drilling conditions. The invention disclosed and claimed herein is directed toward this issue.
The invention features an improved hydraulic system for actuation of a measurement-while-drilling mud valve in which by-pass means provides for reduction of hydraulic load at a desired amount of actuator stroke. The hydraulic system includes a means for driving a hydraulic pump, a hydraulic pump, a valve for switching flow of hydraulic fluid between ports of a hydraulic actuator and a hydraulic actuator operatively coupled an MWD mud valve in which a by-pass means reduces hydraulic loading following a desired amount of actuator stroke. The system may include filtering or straining means, high pressure by-pass means, hydraulic reservoir and/or means to equalize the static pressure of the hydraulic system with the pressure of the surrounding environment.